Caprolactam is the popular monomer for the versatile nylon-6 polyamide, which offers excellent properties like a high strength-to-weight ratio, good chemical and thermal stability, and durability. Textile yarn manufactured from nylon-6 exhibits fine drape, resistance to abrasion, high flexibility, chemical and biological stability, etc. Nylon-6 is widely used in the manufacture of fishing nets, tire yarns, sewing threads, industrial drive belts, etc. As an engineering plastic, it has found a wide application in castings, injection molding, and extrusion. Products manufactured using nylon-6 offer excellent engineering properties, even at high temperatures. The precursor to nylon-6 caprolactam is sold internationally on a very large scale (several billion pounds annually). A competitive and environmentally friendly process of making ε-caprolactam would be extremely desirable.
The ever increasing industrial demand for nylon-6 (polycaprolactam) necessitates the development of environmentally benign methods of producing its precursor, ε-caprolactam. The precursor to nylon-6, ε-caprolactam, is currently manufactured on a massive scale through two favored methods, each of which starts from cyclohexanone. In one, the oxidant that forms the intermediate cyclohexanone oxime is hydroxylamine sulfate, and ammonia is used to neutralize the liberated acid. In the other, a far less environmentally aggressive oxidant, aqueous H2O2 is used in conjunction with a solid redox catalyst, a titanosilicate known as TS-1, to ammoximize the ketone. However, both methods entail the use of oleum to effect the Beckmann rearrangement of the oxime to the lactam and the former method generates very large quantities of low-value ammonium sulfate as by-product.
The avoidance of production of large amounts of low value and unwanted ammonium sulfate would be desirable. So too would it be desirable for production of ε-caprolactam by a method which avoids any use of the highly corrosive fuming sulfuric acid.
It also goes without saying that the process must also have a commercially satisfactory yield level. Current processes approximate about 60% yield for conversion of cyclohexanone to ε-caprolactam. Thus a successful competitive, environmentally friendly process must approximate these yields.
It is a primary objective of the present invention to develop an environmentally friendly (green) one-step catalytic production of ε-caprolactam (precursor of nylon-6) which avoids generation of large quantities of low-value ammonium sulfate as a byproduct, and which avoids use of fuming sulfuric acid, a highly corrosive difficult-to-handle material.
It is a further objective of the present invention to develop such a process which provides conversion of cyclic amines to lactams at competitive yield levels.
The method and means of accomplishing each of the above objectives will become apparent from the detailed description of the invention which follows hereinafter.